Following a deposit-related overheating failure of a smelt spout opening tube in a single-drum Kraft recovery boiler, an investigation was initiated to focus on deposit mechanisms in the waterwall tubing. During the next scheduled outage, a similar spout opening was removed for destructive testing. Highly localized zones of thick deposit were documented inside the composite tube opening, while immediately adjacent areas inside the tubing were clean. The deposit morphology suggests steam pockets formed inside the spout opening tubes because of departure from nucleate boiling (DNB), which is a function of heat flux and circulation. Additional damaged composite tubes from this unit were evaluated over an eight-year period. Nearly all of the problem tubes contained excessive levels of waterside deposits, which led to corrosion thinning of the stainless steel cladding, overheating, and thermal fatigue cracking. Video probe inspections and deposit-weight density (DWD) tests revealed the vast majority of the boiler was clean. There was no evidence that the recurring tube problems were caused by water quality or water treatment issues. A Pitot tube study confirmed poor circulation in a composite spout opening tube during a period of upset operations, which suggests inadequate circulation was the probable root cause for the deposition and tube damage in the lower wall of this unit. Modification of the boiler design did not remedy the problem.
A four-year old Kraft recovery boiler experienced a failure on a smelt spout opening composite tube. The operator observed indications of a small leak and initiated an emergency shutdown procedure (ESP) to avoid a smelt water explosion. Severe cracking and a bulge were found on the furnace side of the tube. No cracking was identified three months earlier during inspection of the opening for surface cracking. The spout opening had been in service for 15 months. A metallurgical investigation identified the failure mechanism to be overheating (creep rupture) caused by thick deposit buildup on the waterside of the spout opening tubes. The DWD values for the cracked areas of the opening tubes were 60-114 g/ft2 (65-123 mg/cm2), and the stainless steel cladding thickness was reduced by nearly 90% due to sulfidation corrosion. The analysis raised concerns about water quality and water treatment in this boiler system. The heavy deposit contained significant amounts of iron, titanium, copper, and calcium phosphate. Extensive steam blanketing was also reported at the damaged spout opening. As a result of these findings, an investigation was initiated to determine the cause for the deposition, and to see if other tubes in the recovery boiler were affected.
System Description
Five boilers operate at this mill with common feedwater and common water treatment, including two 850-psig (5.8-MPa) black liquor recovery boilers and three power boilers. The older recovery boiler is a traditional two-drum design that uses studded carbon steel tubes in the lower furnace. The spout opening tube failure occurred in the newer single-drum recovery boiler, which uses composite tubing in the lower furnace. These composite tubes are comprised of a thin co-extruded layer of Type 304L stainless steel on a carbon steel substrate. When the single-drum recovery boiler was initially put in operation, the boiler water treatment was a chelant-polymer program. Two years after start-up the water treatment was upgraded to a coordinated phosphate program with dispersant to improve corrosion control and to minimize iron deposition. The feedwater for the boiler system is demineralized make-up, with 58% condensate return. The unit uses a sweetwater condenser for attemporation wa
